Vos is a psycho
Vos shows signs of sadism in the first murder. She also must have had complete control when Colin murdered Parse and his daughter because the murders were just as sadistic.
The final murders must have been committed by Vos, too. So when she tells Colin he had been in control all the time, she was lying. She murdered her family blaming it on Colin.
Why? Because she's always dreamed of murdering them?
I also don't understand why Colin had to pick a fight with Parse before butchering him.